The present invention is directed generally to pattern recognition apparatus, also known as and referred to herein as vision systems. More particularly, the present invention is directed to an improvement, by modification, to the pattern recognition apparatus and method of the '205 patent.
The '205 patent is directed to a pattern recognition apparatus and method for measuring displacement (or position) of a target object relative to a reference object using digital techniques. The method and apparatus of the '205 patent employs "signatures" obtained from digital image data of the reference and target objects to compute relative position. Comparison techniques wherein different portions of the reference and target signatures, called "sub signatures", are successively compared are employed to determine the amount of shift, if any, between the reference and target objects and hence provide an indication of the relative position of the target object.
The digital image from which the signatures are generated is a matrix of picture elements called "pixels". Each integer pixel reports a single magnitude representing the intensity of light reflected from a portion of an object viewed by a camera. Thus, an integer pixel cannot report partially one intensity and partially another. Each integer pixel in the matrix contributes to the overall image of the object, but its only contribution is a signal having a magnitude representing a single intensity level.
The aforementioned pixel characteristic gives rise to a problem with the method and apparatus of the '205 patent. That is, the relative position of the target object can be reported only in an integer number of pixels, even if the target object is shifted only a fraction of a pixel ("sub pixel"), or a fractional number of pixels, relative to the reference object. As a consequence, the position data provided by the apparatus of the '205 patent often contains an error; that error is the fraction of a pixel that is missing in the position data.
There is yet another problem with the method and apparatus of the '205 patent caused by the pixel characteristic. Consider FIG. 2, graph 42, which is a graph of the degree of match between the signature of a reference object and the signature of a target object, as the target object is moved through fractional pixel steps. Note that graph 42 is a large amplitude sawtooth having sharp rising and falling edges. The large changes in amplitude in graph 42 occur because integer pixel distances are being compared. Thus, when the target object is displaced exactly 1, 2, 3, or any integer number of pixels, relative to the reference object, the degree of match is good, as indicated by the minima of graph 42 (e.g., minima 43). However, when the target object is displaced a fractional number of pixels relative to the reference object, e.g., 1/2 pixel, 1 1/2 pixels, etc., the degree of match is poor, as indicated by the maxima of graph 42 (e.g., maxima 45). Since, in the method and apparatus of the '205 patent, the degree of match between the two signatures is the key parameter in the decision whether to accept or reject the current "eyepoint" (i.e., scene of the object viewed by the camera), the graph of the degree of match between the two signatures should ideally approximate a horizontal line and not have large changes in amplitude between integer pixel positions. This ideal could be approximated if it were possible to provide more accurate position data, i.e., to within a fraction of a pixel.
It is therefore desirable to provide an improved pattern recognition method and apparatus that reports relative position data in fractional pixels, i.e., reports "sub pixel" positions. It is further desirable that any modifications necessary to the method and apparatus of the '205 patent to achieve the foregoing objective be inexpensive and easy to implement. The present invention meets these objectives.